The present invention relates to a method for manufacturing a steel wire material for reinforcing an optical fiber, the material having on the surface thereof a coating capable of effectively preventing the production of hydrogen.
As a reinforcing material for an optical fiber, steel wire materials such as steel wire or strand and organic high-molecular fibers are used. Among them, since steel wire materials are inexpensive and have a high strength and high rigidity, they are widely used.
But with the steel wire material, because its resistance to corrosion is generally poor, if moisture infiltrates into the optical fiber, corrosion will progress and cause the breakage of wire. Also, it is possible that hydrogen gas is produced as a result of reaction between the steel wire itself and moisture. The hydrogen gas might infiltrate into the optical fiber, thus lowering its light transmission efficiency. Such a problem is peculiar to a steel wire for reinforcing an optical fiber.
It is believed that there is a certain correlation between the corrosion resistance of the steel wire and the amount of hydrogen produced. Thus various methods have been proposed to increase the corrosion resistance of steel wire materials used to reinforce an optical fiber.
One method is to plate a steel wire material with zinc or copper. But, because zinc plating is formed by attaching zinc having a low corrosion potential with respect to steel to preferentially corrode the zinc and thus protect the steel against corrosion, hydrogen gas might be produced if the copper and moisture react with each other as shown by the following formula. EQU Z.sub.n +2H.sub.2 O=Z.sub.n (OH).sub.2 +H.sub.2
This will cause a reduction in the light transmission efficiency of the optical fiber. Thus, this method is effective in protecting the steel against corrosion but not in preventing the production of hydrogen at all.
On the other hand, copper plating is formed by attaching to a surface copper having a high corrosion potential with respect to steel. In this method, if there is a defect in the plating surface such as a pin hole, the steel will begin to corrode from this point. In order to assure a high corrosion resistance, it is necessary to increase the thickness of the plating. But thickening the plating will result in an increase in the plating time, thus lowering the productivity and increasing the cost.
Also it was proposed to coat a steel wire with aluminum, titanium or an alloy thereof to protect the steel wire against corrosion. But with this method, too, the production cost tends to be rather high.
Further, it was proposed to cover a steel wire with a film of organic resin to protect the steel wire against corrosion. But in addition to the fact that organic resins are expensive, some of them produce hydrogen themselves and some allow hydrogen to pass therethrough easily. Further, in order to prevent the formation of pin holes, it is necessary to increase the thickness of the coating. This will further increase the production cost.
Thus none of the conventional anti-corrosion methods was capable of sufficiently restraining the production of hydrogen from a steel wire material used as a material for reinforcing an optical fiber.